Modern sheet fed printing presses, having widths as large as seventy-two inches or more, operate at such high speeds, substrate speeds of 500 feet per minute are common, that the coatings or inks applied to the sheets do not completely dry or cure before reaching the sheet stacking equipment positioned at the ends of the delivery end housings located after such presses. To deal with the problems created by the inability of such inks and coatings to adequately dry or cure before the sheets are stacked one upon another, anti-offset powders, in mixtures of generally small sizes, are often employed to prevent set-off and/or blocking. Set-off is the transfer of ink or coating from the surface of a first printed sheet to the back of the next sheet that falls on top of the first sheet. Blocking is the adhesion of several sheets of a stack of sheets due to the undried ink or coating of sheets sticking to the next adjacent sheets.
Anti-offset powder ranges in size from about five to fifty microns and is usually a starch. The most frequently used anti-offset powders are corn starch and potato starch, but wax sprays, miocroencapsulated particles, and chemically modified starches are also used, and each has certain distinct qualities. Starches may be treated with silicone to agglomerate the fine particles and assist in settling, and silica, tricalcium phosphate, and magnesia may also be added to the powders to improve their flow characteristics.
Anti-offset powder dispensers are positioned in press delivery end housings between driers, which accelerate the drying or curing of the coatings applied to the sheets that pass beneath the driers, and the sheet stackers located adjacent the ends of such housings. The amount of powder used is a function of the sheet weight and the type of ink or coating applied. The lighter the sheet weight, the less powder applied. While the powder that adheres to the non-dried portions of the coated sheets effectively prevents set-off and blocking of the sheets, the excess or airborne powder that does not adhere to the sheets contributes to many problems. A significant amount of the powder, particularly the smaller particles, directed toward the sheets does not adhere to them and remains airborne. It is estimated that about 30% of such powder adheres to the undried coatings of such sheets, about 10% is heavy enough to drift or fall downwardly to the floor, and about 60% remains airborne.
Air currents generated within and adjacent a press delivery end housing by rapid movement of the sheets and their conveying equipment, by the heat created by drying equipment in such housing, and by the descent of the sheets released from the conveyor gripper bars to the top of the stacker create turbulence. In some presses fans are installed adjacent the ends of the sheet conveyors to direct air downwardly on the tops of sheets as they are released from the conveyor gripper bars to prevent sheet fluttering. Such fans create further turbulence. The turbulence causes the airborne powder, unless collected, to circulate throughout pressrooms and ultimately deposit everywhere. The airborne powder contributes to sheet quality problems by reducing the gloss on the printed surfaces, by creating scratches and hickeys on such surfaces, and by generally causing dirty prints. The powder also penetrates into all portions of pressroom equipment, substantially increasing maintenance and cleaning costs and contributing to health and safety problems. In addition to airborne powder problems in pressrooms, the inks and coatings used in printing presses generate undesirable mists and fumes which prevade the atmosphere and create undesirable conditions.
The problems with airborne powder in high speed presses is complicated by the fact that presses come in different sizes, both with respect to the number of printing stands and the widths and lengths of the delivery end housings, print sheets of different lengths, widths and weights, operate at different speeds and are made by different manufacturers. Manufacturers of printing presses emphasize the speed of operation and mechanical features of their presses but, to date, have done little to reduce the environmental problems associated with their presses.
Various forms of apparatus, methods, and systems have been proposed and tried for dealing with the problems resulting from the use of inks, coating materials, and anti-offset powders in the operations of high-speed printing presses. For example, in some pressrooms, hoods, connected to a vacuum system, have been placed above the ends of the delivery end housings of a number of presses to collect excess, airborne powder. Gates or dampers placed in the ducts between each of the hoods and the vacuum system are adjusted to control the degree of suction within the hood. In addition, many patented inventions have been proposed to deal with such pressroom problems.
U.S. Pat. No. 3,434,416 to A. O. Testone describes a printing press excess-powder collector that provides a housing about a powder dispensing nozzle which is located above coated sheet material moving through the housing. Excess powder is removed from the housing by a blower or fan that draws such powder and air through a collection compartment in which the powder is electrostatically removed from the air.
U.S. Pat. No. 3,680,528 to R. C. Sanders is directed to apparatus for removing airborne powder following the application of powder material to a moving surface. A manifold for collecting the airborne powder is positioned after the powder dispensing apparatus. The powder collected in the manifold is moved by a fluid stream to the ends of the manifold where it is exhausted by a gentle vacuum to an exhaust trap.
U.S. Pat. No. 3,882,818 to K. D. Mowbrey is directed to a system for collecting excess airborne powder from powder applied to the surfaces of sheets passing from a printing press. The collecting apparatus is located adjacent and following a powder applicator and comprises a plurality of modules that extend transversely of the direction of travel of such sheets, between the sheet conveyor flights and above the upper flight. The modules have inlet openings along their bottoms and/or one side thereof and gates to vary the size of the openings. The excess powder and air are drawn into the module openings which are connected by conduits to an exhaust manifold which is supported on a plate positioned above the conveyor upper flight.
U.S. Pat. No. 2,710,574 to H. E. Runion is directed to apparatus for eliminating undesirable dust or ink mist from pressrooms. Each press in a pressroom is surrounded by an enclosure having a plurality of small openings adjacent the top of the enclosure. Each enclosure is connected by a first passage to a plenum chamber which communicates through a filter with a second plenum chamber. The second chamber communicates through a second passage with a blower or fan. The fan draws air through the enclosure openings, the enclosure, the first passage and through the filter separating the plenum chambers, through the second passage, and discharges the air through a third passage to a desired location.
U.S. Pat. No. 4,563,943 to M. H. Bertelsen et al. is directed to apparatus for filtering the air passing from the top discharge opening of a computer room for housing a high-speed paper-handling machine, such as a laser printer. The apparatus includes a sheet metal wing that extends over the top opening of the housing and connects with filter apparatus. The wing has a bottom inlet opening that faces the housing top opening to receive air from the printer. The wing includes a longitudinally extending baffle that extends between the wing sidewalls and divides the wing inlet opening into upper and lower inlet portions of equal size.
U.S. Pat. No. 4,875,054 to T. H. V. Archer is directed to a somewhat different invention that makes use of an air curtain in fluid jet printing apparatus. The patent discloses a clean air hood for disposition directly above the print head of a fluid jet printing device. The device has an orifice plate on one side of the print head for forming a plurality of droplets that flow downwardly past charge and deflection electrodes and a fluid catcher. The charge electrode imparts a charge to certain droplets which are caught on the fluid catcher for recycling, while the uncharged droplets are deposited on a substrate beneath the electrodes and the fluid catcher. The hood includes an interior baffle to direct clean air from above the hood downwardly through the hood so that it passes through spaced slots on opposite sides and opposite ends of the baffle to form descending air curtains that straddle the print head and minimize the tendency of the droplets contacting the electrodes.